1. Field of the Invention
The claimed subject matter relates to methods and compositions involving high energy absorption syntactic metal foams containing substantially uniformly distributed ceramic microballoons in a metallic matrix.
2. The Related Art
Various syntactic metal foams and methods for forming them had been proposed. Foamed materials wherein the pores are formed by the inclusion of hollow microballoons are generally described as syntactic foams. The size, volume fraction, and uniformity of the syntactic porosity are determined by the included microballoons.
One type of syntactic metal foam is exemplified by Rohatgi U.S. Pat. No. 5,899,256 wherein it is proposed, inter alia, that a mass of nickel coated cenosphere fly ash particles may be infiltrated with molten aluminum to form an aluminum-cenosphere fly ash composite. The nickel coating on the fly ash is said to enhance the wetting of the fly ash by the molten matrix metal, thereby reducing the pressure required to force the molten metal through the body of fly ash. Rohatgi acknowledges that melt infiltration results in non-uniform properties in the composite, apparently because the molten metal washes the nickel coating off the fly ash.
Rohatgi U.S. Pat. No. 5,228,494, which is hereby incorporated herein by reference as though fully set forth hereat, also proposes to use fly ash in the production of cast metal-fly ash composites. This is a melt infiltration process. It is stated, without elaboration or exemplification that such cast metal-fly ash composites can be subsequently hot worked and/or cold worked.
Inabata U.S. Pat. No. 4,939,038, which is hereby incorporated herein by reference as though fully set forth hereat, proposes, inter alia, combining coated hollow microspheres with a matrix forming powder, and heating the mixture to form a composite.
Nakao U.S. Pat. No. 3,781,170, which is hereby incorporated herein by reference as though fully set forth hereat, proposes the production of lightweight metal composite material from a mixture of hollow microspheres in a light metal matrix powder. Heat and pressure are applied to the mixture. The pressure is continuously adjusted as the mixture shrinks during processing.
The properties of many sinterable materials, such as melting points, softening points, onset sintering temperatures, and liquidus and solidus points are well known and available from various published sources. Well known conventional procedures are available for determining such properties where they are not readily available in published references.
Conventional solid state processing of powdered metals usually involves heating a green preform to at least the onset sintering temperature of the matrix material, but often below any temperature at which a liquid phase forms. Conventional powdered metal consolidation often takes place without the formation of a liquid phase. Transient liquid phase sintering is a solid state process that involves heating the powdered matrix forming metals to a temperature at which a liquid phase initially appears, but then disappears before sintering is completed. There is no detectable solid phase corresponding to the transient liquid phase in the final product. See, for example, Sherman et al. U.S. Pat. No. 7,041,250, which is hereby incorporated herein by reference as though fully set forth hereat. In conventional liquid phase sintering, a liquid phase forms during sintering and does not disappear, so there is a solid phase corresponding to the liquid phase in the final compact.
The use of inorganic non-metallic (ceramic) microballoons in a syntactic metal foam had generally been considered to require careful handling of this foam during and after its formation to avoid crushing the microballoons. Such crushing is known to increase the density of the foam in a non-uniform and uncontrolled way by eliminating or diminishing the syntactic porosity to an unpredictable degree within various random regions of the foam. This randomness renders the properties of the foam unpredictable, and, therefore, unreliable.
Unconstrained ceramic microballoons have measurable average unconstrained uniaxial crush strengths. Previously, it had generally been understood that the deformation processing of syntactic metal foam composites at or above the average unconstrained uniaxial crush strength of the included ceramic microballoons would crush enough of the ceramic microballoons to have a significant adverse effect on the physical properties of the final syntactic metal foam composite product. This had substantially limited the use of deformation processing in shaping or achieving desired physical properties in syntactic metal foam composites.
There was a well recognized and longstanding need for syntactic metal foam composites that can be deformation processed at or above the average unconstrained uniaxial crush strength of the included ceramic microballoons. There was also a well recognized and longstanding need for syntactic metal foam composite forming methods that would allow such composites to be shaped without fracturing or otherwise impairing their suitability for use.
The density of syntactic metal foam decreases as the rate of ceramic microballoon loading increases for a given average ceramic microballoon size. Also, for a given loading rate the density of syntactic metal foam composites decreases as the wall thickness of the included ceramic microballoons decreases. Previously, it had been generally understood that for a given average particle size many of the desirable physical properties of syntactic metal foam composites tended to decrease as the rate of ceramic microballoon loading increased. Further, it had also been understood that for a given average particle size many of the desirable physical properties of syntactic metal foam composites tended to decrease as the wall thickness of the included ceramic microballoons decreased. There was a well recognized and long standing need for lightweight syntactic metal foam composites with high thin walled ceramic microballoon loading rates.
There was a well recognized need for methods whereby syntactic metal foam composites could be formed with substantially uniformly distributed closed cells, and closely controlled sphere volume fraction. Also, there had been a well recognized need for high strength syntactic metal foam composites with substantially uniform and predictable physical properties. Such physical properties include, for example, ductility, high stiffness, and yield strengths at densities of, for example, from 40 to 65 percent of those of the pore free matrix metals.
There was a need for ductile, porous, closed-cell metallic structures having a uniform and controlled distribution of matrix metal and empty (pore) phases. Further, there was a need for a method of producing a reduced density metallic material that could be substituted or used for energy adsorption, ballistic and blast protection, lightweighting applications, EMI shielding, packaging, and structural applications. Previous expedients generally offered, for example, poor control over microballoon packing and distribution, or limited strength, or lack of uniform ductile properties, or relatively high densities, or poor workability or some combination of these.
In the event of a conflict of any nature between the teachings of references incorporated herein by reference, and the teachings of this disclosure, it is intended that the teachings of this disclosure shall control.
These and other shortcomings of the prior art have been overcome according to the present invention. There is a need for the present embodiments that enable a highly uniform distribution of ceramic microballoons with excellent control over sphere volume fraction, while enabling syntactic metal foam composite embodiments with desirable, uniform, predictable properties to be produced. Embodiments are capable of being deformation worked at or above the average unconstrained uniaxial crush strength of the included ceramic microballoons. Such embodiments exhibit, for example, ductility, excellent strength, compression loading, energy absorption, workability, and low density properties.